first of all they're the most simple (and yet the most effective) life form, and therefore it's only logical they can represent one of the first steps in the evolution grand scheme (some scientists believe they evolved from a different branch of cells that sort of regressed tho): specifically the RNA ones are the oldest, since oxygen lacked in the old-earth atmosphere and that favoured RNA over DNA. Still, they cannot be older than cells themselves, since they need to use them to "reproduce". Very often some viral genetical mark can be traced way back in old DNA as well, and that can give you some sort if indication of the pathways the virus took to get the genetical configuration it has today, but apart from that it's really hard to put a pin in time.

Sorry for my english, it's not my first language